How to conduct code audit effectively?

For most of the software and applications, code review is an excellent way to keep them healthy and in working manner but the harder part is to find code review techniques. However, most of the websites nowadays offer all these code review methodologies but there are various things which need to be taken care of before utilizing any of them. But the question arises about how to conduct code audits effectively, any suggestions?

The Combined Code requires that audit committees conduct an annual evaluation of the effectiveness of the audit process. Process based auditing is recognized as the best practice method for effective audits. In order for this to be done properly, committees need to determine what they mean by effectiveness, and to implement appropriate procedures for the evaluation.